The Slope (Spatial Analyst or 3D Analyst) tool calculates the slope of the terrain, either in degrees or percentage rise.
To calculate the slope of the terrain (Fig. 39):
- Turn on Spatial and 3D Analyst extensions, go to Customize (from the main menu).
- Select Extensions.
- Check the Spatial Analyst (and/or 3D Analyst) extension.
- Click Close.
- Type “slope” into the Search window (or browse to Spatial Analyst Tools > Surface > Slope in the ArcToolbox).
- Select the Slope (Spatial Analyst or 3D Analyst) tool.
- Choose the DEM as the input raster (aster30_frib_ch.tif).
- Save the output raster as “slope”.
- Choose the output measurement to be used, here we will use degrees.
- If the X, Y, and Z (altitude) coordinates are all in meters, leave the Z factor as 1. Otherwise click the Show Help button in the tool to read about how to convert the Z factor.
- Click OK.
- Turn off all other layers in the TOC, right-click Layer at the top of TOC.
- Select Turn all layers off.
- Turn on the new slope raster in the TOC.
The results show the default classification scheme in degrees ranging from 0 to about 58 degrees in eight categories (Fig. 40).
Now the results will be classified in a different way by opening the layer properties (Fig. 41).
- Right-click slope layer in the TOC.
- Select Properties.
- Select the Symbology tab.
- Choose the “stretched” option in the Show dialog box.
- Select a colour ramp.
- Choose the stretch type of Minimum-Maximum.
- Click OK.
The results (Fig. 42) show that the region is not so diverse in terms of slope.
Fig. 39. The Slope tool dialog box.
Fig. 40. The result of slope calculation from the DEM.
Fig. 41. The symbol properties dialog box for the calculated slope layer.
Fig. 42. The results of changing the symbology of the slope layer.